1. Field of the Invention
The present invention is directed to wireless communication systems and, more specifically, to the transmission of acknowledgement signals in response to the reception of respective data packets.
2. Description of the Related Art
The DownLink (DL) of a communication system transmits signals from a serving base station (Node B) to User Equipments (UEs) over an operating Bandwidth (BW). The DL signals include data signals that provide the data information, control signals that provide control information for the scheduling of data signals, and Reference Signals (RS), also known as pilot signals, that enable coherent demodulation of data or control signals. The DL data signals are transmitted through the Physical Downlink Shared CHannel (PDSCH).
The UpLink (UL) of a communication system transmits signals from UEs to their serving Node B. The UL signals also include data signals, control signals and RS. The UL data signals are transmitted through the Physical Uplink Shared CHannel (PUSCH). In the absence of PUSCH transmission, a UE transmits its UL Control Information (UCI) through the Physical Uplink Control CHannel (PUCCH); otherwise the UE may transmit the UCI through the PUSCH.
A UE, also commonly referred to as a terminal or a mobile station, may be fixed or mobile and may be a wireless device, a cellular phone, a personal computer device, etc. A Node B is generally a fixed station and may also be referred to as a Base Transceiver System (BTS), an access point, or some other terminology.
An exemplary multiplexing method for DL signal transmissions is the Orthogonal Frequency Division Multiple Access (OFDMA), and an exemplary multiplexing method for UL signal transmissions is the Single-Carrier Frequency Division Multiple Access (SC-FDMA), as they are also considered in the 3GPP Long Term Evolution (LTE). These multiplexing methods serve only to illustrate applications and are not restrictive to the present invention.
DL control signals transmitted through the physical layer may be of broadcast or UE-specific (unicast) nature. Broadcast control signals convey system information to all UEs. The system information may be transmitted in different broadcast channels having different transmission rates depending on how quickly the broadcast control information should be obtained by the UEs. For example, a Broadcast CHannel (BCH) may consist of a Primary BCH (P-BCH) and a Secondary BCH (S-BCH). UE-specific control signals convey Scheduling Assignments (SAs), for PDSCH reception (DL SAs) or PUSCH transmission (UL SAs) by UEs, and ACKnowledgement (ACK) and Negative Acknowledgement (NAK) signals associated with the use of Hybrid Automatic Repeat reQuest (HARQ) for PUSCH transmissions (HARQ-ACK signals). The Node B transmits to a UE a HARQ-ACK signal with a positive (ACK) or negative (NAK) information value, in response to a correct or incorrect PUSCH reception, respectively. The Node B transmits the HARQ-ACK signals through the Physical Hybrid-ARQ Indicator CHannel (PHICH). The DL SAs, the UL SAs, the PHICH, and possibly other control channels, are conveyed from the Node B to UEs through the Physical Downlink Control CHannel (PDCCH).
An exemplary PDCCH transmission structure in the DL Transmission Time Interval (TTI), which for simplicity is assumed to consist of one sub-frame having M OFDM symbols, is shown in FIG. 1. The PDCCH 120 occupies the first N OFDM symbols 110. The Node B informs the UEs of the PDCCH size through the transmission of a Physical Control Format Indicator CHannel (PCFICH) in the first OFDM symbol (not shown for simplicity). The remaining M-N OFDM symbols are primarily used for PDSCH transmission 130. The PHICH 140 is transmitted in some PDCCH sub-carriers, also referred to as Resource Elements (REs), which may be placed only in the first PDCCH symbol or in all PDCCH symbols as in FIG. 1. Some OFDM symbols also contain RS REs, 150 and 160, for each of the Node B transmitter antennas which in FIG. 1 are assumed to be two. The PHICH REs are grouped in consecutive REs with only RS REs possibly being placed between PHICH REs. Each group of PHICH REs consists of 4 REs and will be referred to as Resource Element Group (REG). A group of 12 consecutive REs 170 will be referred to as a Physical Resource Block (PRB). For the present example, both the DL BW and the UL BW include PRBs and the respective PDSCH and PUSCH transmissions occur over an integer number of PRBs. For example, an UL BW of 18 MHz consists of NRBUL=100 PRBs of 180 KHz with the PRBs indexed from 0 up to NRBUL−1.
The PHICH resource used for an HARQ-ACK signal transmission is assumed to be linked to the PRBs used for the respective PUSCH transmission. Therefore, the PHICH resources depend in principle on the total number of PRBs, NRBUL, in the UL operating BW. When multiple PRBs are allocated to a PUSCH transmission, the PHICH resource is determined from the PRB with the lowest index IPRB_RAlowest_index (first PRB for a PUSCH transmission).
Spatial Division Multiple Access (SDMA) is an effective technique for improving UL spectral efficiency. With SDMA, some PRBs are shared by PUSCH transmissions from multiple UEs. SDMA is facilitated by providing orthogonal RS to the respective UEs so that the Node B can obtain an accurate estimate for the channel response experienced by each PUSCH transmission. Using SC-FDMA for PUSCH transmissions, the RS is assumed to be constructed from a Constant Amplitude Zero Auto-Correlation (CAZAC) sequence. Orthogonal RS can then be obtained by applying different Cyclic Shifts (CS) to the CAZAC sequence representing the RS, or by applying Orthogonal Covers (OC) in the time domain in case of 2 or more RS in the PUSCH. Each UE is informed of the CS, OC, or both, to apply to a CAZAC sequence for the RS transmission in the PUSCH through the CS Indicator (CSI) Information Element (IE) provided in the UL SA (the CSI may also indicate a OC used together with a specific CS).
Since PUSCH PRBs are shared among multiple SDMA UEs, using only IPRB_RAlowest_index to determine the PHICH resource for each respective HARQ-ACK signal transmission may lead to collisions as the same PHICH resource may correspond to multiple SDMA UEs if they have the same IPRB_RAlowest_index for their PUSCH transmissions. This problem is avoided by having the CSI IE serve not only to assign the CS, or OC, or both, for the RS transmission in the PUSCH but also for adjusting the resource for the respective PHICH transmission.
FIG. 2 illustrates the use of the CSI IE for adjusting the resource of a PHICH transmission where the UL operating BW consists of NRBUL=100 PRBs 210, there are 4 SDMA UEs, and the PUSCH transmission BW is 10 PRBs with the lowest PRB index being equal to 8 220. Assuming that the CSI IE consists of 3 bits, the 8 CSI values can map to 8 respective incremental shifts, including a zero shift, for the PHICH resource relative to the one obtained from IPRB_RAlowest_index. By respectively assigning to the first, second, third, and fourth UEs the first four CSI values, CSI 0 232, CSI 1 234, CSI 2 236, and CSI 3 238, the respective PHICH indexes are PHICH 1=8 242, PHICH 2=9 244, PHICH 3=10 246, and PHICH 4=11 248. This approach requires that the number of SDMA UEs is less than or equal to the number of PRBs assigned to their PUSCH transmissions which is typically the case in practice.
It is desirable that each HARQ-ACK signal is not confined in only one RE but is instead spread over all REs in each REG to obtain interference randomization. To avoid reducing the multiplexing capacity of the PHICH (by a factor of 4 in FIG. 1), orthogonal multiplexing of the PHICH may apply within each REG using, for example, Walsh-Hadamard (WH) orthogonal sequences with Spreading Factor (SF) of NSFPHICH (where, in FIG. 1, NSFPHICH=4). For Quadrature Phase Shift Keying (QPSK) modulation and HARQ-ACK signals conveying a binary value (ACK or NAK), each PHICH channel may be placed on the In-phase (I) or Quadrature (Q) QPSK component and be further modulated with an orthogonal sequence over each REG. For example, for a REG consisting of 4 REs and orthogonal sequences with NSFPHICH=4, the PHICH multiplexing capacity (PHICH resources) for HARQ-ACK signals with binary values is 2NSFPHICH=8 (obtained from a factor of 2 from the I/Q dimensions of QPSK times a SF of NSFPHICH=4 from the number of orthogonal sequences over the REG of 4 REs).
FIG. 3 illustrates a HARQ-ACK signal transmission from the Node B in one of the PHICH resources available within one REG consisting of 4 REs. An HARQ-ACK bit 310 is multiplied in multipliers 322, 324, 326, and 328, by each element of the WH sequence 332, 334, 336, and 338 and the resulting output is placed on the I-branch of the QPSK modulated RE 342, 344, 346, and 348 (the Q-branch may be used for the HARQ-ACK bit for another UE). The WH sequence may be one of the 4 WH sequences 350. With I/Q multiplexing and orthogonal sequence multiplexing with NSFPHICH4,8 PHICH channels are provided within one REG. The UE receiver needs to only perform the conventional functions of QPSK demodulation and orthogonal sequence despreading (and averaging over the repeated PHICH group transmissions as discussed below).
The HARQ-ACK signal transmission in each PHICH group may be repeated over multiple REGs to obtain frequency diversity and improve the effective Signal-to-Interference and Noise Ratio (SINR).
FIG. 4 illustrates the repetition for the HARQ-ACK signal transmission in 3 PHICH groups, 412, 414, and 416, over 3 respective REGs, {422, 424, 426}, {432, 434, 436}, and {442, 444, 446} in the same OFDM symbol (different OFDM symbols may also be used as in FIG. 1). The number of symbols used for the PHICH transmission defines the duration of the PHICH transmission which can be indicated to UEs through the P-BCH. For example, a 1-bit value in the P-BCH can indicate whether the PHICH transmission is in 1 or 3 OFDM symbols.
Multiple PHICH resources mapped to the same set of REs in one or more REGs constitute a PHICH group. PHICH resources in the same PHICH group are separated through I/Q multiplexing and through different orthogonal sequences. A PHICH resource is identified by the index pair (nPHICHgroup, nPHICHseq), where nPHICHgroup is the PHICH group number and nPHICHseq is the orthogonal sequence index within the group. The number of PHICH groups is given by NPHICHgroup=┌Ng(NRBDL/8)┐ where Ngε{⅙,½,1,2} is a parameter identified to UEs through the P-BCH and the ┌ ┐ operation rounds a number to its next integer. It is assumed that the total number of DL PRBs, NRBDL, is known by the UEs prior to any PHICH reception while the total number of UL PRBs, NRBUL, may not be known. For this reason, NRBDL (not NRBUL) is used to specify NPHICHgroup. The PHICH group number is determined by Equation (1).nPHICHgroup=(IPRB_RAlowest_index+CSI)mod NPHICHgroup  (1)The orthogonal sequence index within the group is determined by Equation (2)nPHICHseq=(└IPRB_RAlowest_index/NPHICHgroup┘+CSI)mod 2NSFPHICH  (2)In Equation (2), the └ ┘ operation rounds a number to its previous integer. PHICH resources corresponding to consecutive PRBs are mapped to different PHICH groups.
In order to support higher data rates than possible in legacy communication systems, for aggregation of multiple Component Carriers (CCs) is typically considered in both the DL and UL of the communication system to provide higher operating BWs. For example, to support communication over 100 MHz, aggregation of five 20 MHz CCs can be used. For ease of reference, UEs capable of operating only over a single CC will be referred to as “legacy-UEs” while UEs capable of operating over multiple CCs will be referred to as “advanced-UEs”. From a set of multiple DL CCs or UL CCs, an advanced-UE may be assigned PDSCH reception or PUSCH transmission, respectively, only in a sub-set of DL CCs or UL CCs.
FIG. 5 further illustrates the principle of CC aggregation in the exemplary case of DL CCs. This principle can be extended in the same manner for UL CCs. An operating BW of 100 MHz 510 is constructed by the aggregation of 5 (contiguous, for simplicity) DL CCs, 521, 522, 523, 524, 525, each having a BW of 20 MHz. As for the sub-frame structure for a single DL CC in FIG. 1, the sub-frame structure in the case of multiple DL CCs consists of a PDCCH region, such as for example 531 through 535, and a PDSCH region, such as for example 541 and 545. The PDCCH region size may vary per DL CC and its value is signaled by the respective PCFICH. For CCs 1 and 5, the PDCCH size is respectively, PDCCH-1=3 symbols 531 and PDCCH-5=1 symbol 535. Since the PDSCH size is found by subtracting the respective PDCCH size from the sub-frame size, PDSCH-1=11 symbols 541 and PDSCH-5=13 symbols 545.
FIG. 5 also illustrates the extension of the PDCCH design for SA transmissions to advanced-UEs. Scheduling is assumed to be independent among CCs and each PDCCH is contained within one CC regardless of the number of CCs an advanced-UE may use for PDSCH reception or PUSCH transmission. The advanced-UE 550 receives two distinct SAs, SA2 552 and SA3 553, for respective PDSCH reception in the second and third CCs, while the advanced-UE 560 receives SA5 565 for PDSCH reception in the fifth CC. Different Transport Blocks (TBs) are associated with different SAs. Each SA scheduling PDDCH reception in a DL CC or PUSCH transmission in an UL CC that is either linked to the DL CC with the SA transmission or indicated by the SA, may be transmitted in the same DL CC or in a different DL CC.
The examples used herein consider a communication system using CC aggregation and investigates aspects regarding the mapping of PHICH resources. Having a variable number of DL CCs and UL CCs configured for an advanced-UE necessitates a different mapping for the PHICH resources for advanced-UEs relative to the one for legacy-UEs. Moreover, as the DL operating BW may be substantially greater than the UL operating BW, it is desirable that the dimensioning of PHICH groups is not based on the total number of DL PRBs. Also, unlike legacy-UEs for which PUSCH transmission is assumed to be limited to one CodeWord (CW) or one TB, resulting in one respective HARQ-ACK information bit in the DL, transmission of two CWs or two TBs, each using a separate HARQ process, may apply for advanced-UEs having two transmitter antennas through the application of Spatial Multiplexing (SM). Then, support for two HARQ-ACK information bits is required.
Therefore, there is a need to map PHICH resources for advanced-UEs having multiple configured DL CCs and UL CCs.
There is also a need to avoid over-dimensioning the number of PHICH groups in order to avoid unnecessarily increasing the respective DL overhead.
There is also another need to efficiently support 2-bit HARQ-ACK signal transmission while avoiding PHICH collisions.